Recently, the application of high-strength steels to automobiles has increased for the purpose of weight reduction. However, high-strength steels are easily fractured when processed at room temperature. In addition, since high-strength steels undergo spring back during forming processes, it is difficult to precisely determine the dimension high-strength steels so as to manufacture complicated parts. Due to these reasons, hot press forming (HPF) has become favored as a method for shaping high-strength steels.
HPF is a method of forming steel sheets into complicated shapes at high temperatures at which steel sheets become soft and ductile. In detail, HPF is performed by heating a steel sheet to an austenite temperature range or higher, pressing the steel sheet, and rapidly cooling the steel sheet simultaneously with the pressing so as to cause the steel sheet to undergo transformation into martensite. In this manner, precise high-strength products may be manufactured by HPF.
However, if steel is heated to a high temperature, surface corrosion or decarbonization may occur. Thus, steel plated with a Zn-based or Al-based material is widely used in an HPF process. In particular, galvanized steel sheets having Zn-based plating layers are highly resistant to corrosion because zinc (Zn) provides sacrificial corrosion protection to the steel sheets.
A hot-dip galvanized steel sheet disclosed in Japanese Patent Application Laid-open Publication No. 2006-022395 is an example of such steel sheets resistant to corrosion. If a steel sheet is subjected to a hot-dip galvanizing process and then an HPF process, an alloy having a zinc content of 70% or greater is formed on the steel sheet, and thus the corrosion resistance of the steel sheet is improved.
However, if a galvanized steel sheet such as a hot-dip galvanized steel sheet is heated in high-temperature air, zinc oxides are formed on the galvanized steel sheet, and after a forming process, such zinc oxides function as a resistor hindering the flow of current during a welding process such as a spot welding process, thereby worsening the weldability of the galvanized steel sheet. Due to this reason, additional processes may be performed after a forming process so as to remove zinc oxides from such a galvanized steel sheet and thus to improve the weldability of the galvanized steel sheet during a spot welding process. However, this increases manufacturing costs.
A method of preventing the formation of oxides on a steel sheet after an HPF process is disclosed in U.S. Pat. No. 6,296,805. According to the disclosed method, a steel sheet is plated with aluminum (Al) so as to be used as a steel sheet for HPF. If a steel sheet is plated with aluminum (Al) having a high degree of heat resistance and is then subjected to an HPF process, a product formed by the HPF process does not have surface oxides or has very low amounts of surface oxides, and thus the spot weldability of the product is very high. However, unlike zinc (Zn), aluminum provides very insufficient sacrificial corrosion protection to a base steel sheet, and thus if the iron surface of the base steel sheet is exposed, the base steel sheet may markedly corrode.